The present invention relates to an optical instrument and a conveyor associated to provide undistorted, sharply focussed images simultaneously from different angular locations about an object being conveyed, and particularly relates to an inspection machine for visually inspecting the assembly of the components of a circuit board during manufacturing.
In various manufacturing processes, it is frequently necessary and desirable to automatically optically inspect manufactured components for accuracy in their location and structure. For example, in the manufacture of circuit boards, it is customary to optically inspect the three-dimensional structure of the components of the circuit boards as they pass through the manufacturing process. It will be appreciated that circuit boards are generally planar but that the leads of the circuit board components and connecting.(solder) surfaces are generally very small and fully three-dimensional. Thus, circuit boards are conventionally conveyed through an inspection station for this purpose and typically by a conveyor which grasps the opposite side margins of the board to convey the board. It has been difficult, however, to accurately assess the correct assembly of very small, for example, 5 to 10 mm components, with existing optical equipment. A principal cause of this difficulty is in the inability of such existing equipment to eliminate perspective distortion of the optical image and obtain uniform imaging of an angled surface of the object. Moreover, it has been difficult to obtain undistorted perspective images of the object from different angular locations about the object with uniform focus, brightness and magnification and particularly in a light, rigid and compact optical inspection head capable of inspecting all areas of the circuit board It will be appreciated that circuit boards may range in size from 1.times.1 to about 20.times.20 square inches. These difficulties are further compounded by the distortion in the image caused by the weight of the board and its components when the board bows or sags between its oppositely supported marginal edges It will be appreciated that all portions of the board do not therefore present a surface at all times in a horizontal plane correctly spaced relative to the lens of an optical instrument located close to the board to inspect these portions of the board. Efforts to compensate for this sag or bow or, more generally, the lack of coplanarity of the observed portion of the board and a plane normal to the axis of the lens of the optical instrument have resulted in the necessity for complicated computer processing of the resultant images.
In accordance with the present invention, there is provided an optical instrument, e.g., a vision head and a conveyor, which comprise an optical inspection machine for inspecting small objects, or small portions of larger objects, on the conveyor in a manner in which undistorted perspective images of the portions of the object from different angular locations about the object are simultaneously obtained for analysis and comparison with predetermined information regarding the structure of that portion of the object. To accomplish this, there is provided a vision head mounted on a carriage for traversing movement in a plane, for example, in an X-Y plane, relative to a generally parallel plane in which the object under observation is conveyed. The vision head includes a wide angle lens, a plurality of collecting mirrors, a reversing mirror, illumination sources for illuminating the object and sensors for transposing the light signals of the images into electrical signals which may be digitized for general analysis and comparison with predetermined information regarding that structure of the object.
More particularly, the vision head includes a wide angle lens, the axis of which passes normal to the plane of conveyance of the object under observation. Radially disposed about and spaced from the axis and between the lens and the object are a plurality of collecting mirrors. In a preferred embodiment, light-emitting diodes are arranged below the collecting mirrors for illuminating the object. Accordingly, light rays representing the image of the object from different angular locations about the wide angle lens axis are reflected from the collecting mirrors through the wide angle lens. A reversing mirror receives the light rays from the wide angle lens and reflects those rays onto sensors for conversion into electrical signals for later digitization and analysis Thus, the vision head creates a plurality of crisp images of a single three-dimensional region under observation where each image view is at a large angle to the lens axis, e.g., 25.degree. to 45.degree., and at different angular locations about the lens axis, e.g., 0.degree. to 360.degree.. Moreover, each image is uniformly focussed without perspective distortion and with uniform brightness and uniform magnification. Hence, all three-dimensional surfaces of the object are clearly in focus and undistorted. Moreover, the vision head or carriage may be fabricated as a single machined part, with only 0.degree. and 90.degree. machining operations thereby enabling inexpensive precision fabrication of the vision heads. The optical arrangement is also compact and light, allowing rapid movement of the vision head relative to the object under observation and in a very small space.
More particularly, circuit boards, when conveyed along a plane of conveyance, will normally sag or bow along the middle. The sag or bow oftentimes exceeds the depth of focus of automated optical inspection equipment, particularly when viewed at an angle from the vertical, as is necessary for surface measurements and for solder shape examination. Additionally, this sag or bow may displace images to the extent that particular points of the portion of interest are not contained in the image. To compensate for the the irregular spacing and lack of planarity between the object under observation and a plane normal to the axis of the lens, i.e., to compensate for a bow or sag in the circuit board whereby an angle is extant between the plane of conveyance and the surface of the board at the area of observation, a conveyor according to the present invention provides for relative movement of the conveyor sections supporting the margins of the object generally in a direction parallel to the axis of the lens. Where the object sags or bows between the margins, it will be appreciated that, by displacing one of the margins of the object under observation relative to the other margin, the small discrete area under observation may obtain a position substantially parallel to a plane normal to the axis of the lens. By clamping the board at the margins and displacing one margin relative to the other margin in a direction parallel to the axis of the lens, the level or tilt of the circuit board surface at each observable location between the margins may be adjusted into coplanar relation with a plane normal to the axis of the lens.
An additional benefit is that the object can be moved vertically so all images are of the same region of the object, which increases inspection speed.
More particularly, each side section of the conveyor is provided with upper and lower clamping plates. An endless-loop conveyor belt is disposed between the clamping plates The clamping plates are pivoted about a common horizontal axis generally parallel to the direction of conveyance. When a circuit board is introduced into the conveyor, the belt advances the board between the clamping plates. The board is then held fixed, preferably horizontally, between each conveyor section, while the vision head moves in an X-Y plane about and above the board for observing predetermined portions of the board. Each side section may be adjusted independently relative to the other side section to adjust the tilt and level of the surface of the board then under observation by the vision head. Particularly, linear actuators pivot the clamping elements about the pivot axis to effect the relative displacement of the side conveyor sections relative to one another. Additionally, horizontal linear actuators may be used to adjust the width of the conveyor sections relative to one another to accommodate different widths of circuit boards.
In a preferred embodiment according to the present invention, there is provided an optical device for providing discrete, undistorted and uniform images of an object or a small region of an object lying in a plane and having three-dimensional surfaces, with each image taken at a different angular location about an axis passing perpendicularly through the plane and object, comprising a wide angle lens spaced from the object and disposed along the axis, a plurality of collecting mirrors spaced from and disposed at predetermined angular locations about the axis, the mirrors lying in planes generally parallel to the axis and for disposition between the wide angle lens and the object for reflecting light rays from the object through the wide angle lens and means for receiving the light rays passing through the wide angle lens from the object and the collecting mirrors representing an undistorted image of the object as viewed from each angular location with uniform focus of the three dimensional surfaces of the object.
In a further preferred embodiment according to the present invention, there is provided a conveyor for conveying an object having surface portions in a first plane and displacing such surface portions into a second, different plane, comprising first and second conveying sections spaced one from the other with each section including means for engaging marginal portions of an object spanning between the sections for conveying the object in a predetermined direction of conveyance and means carried by each of the first and second conveyor sections for displacing the respective margins of the object in a direction generally normal to the direction of conveyance and independently of one another whereby a specified portion of the object in the first plane may be disposed in the second plane.
In a further preferred embodiment according to the present invention, there is provided a visual inspection machine for visually inspecting surface portions of generally planar objects comprising a conveyor for conveying the planar objects with each planar object having general surface portions in a first plane and displacing such surface portions into a second, different plane, the conveyor including first and second conveying sections spaced one from the other with each section including means for engaging a marginal portion of the planar object spanning between the sections for conveying the element in a predetermined direction of conveyance, means carried by each of the first and second conveyor sections for displacing the respective margins of the element in a direction generally normal to the direction of conveyance and independently of one another whereby a specified surface portion of the planar object in the first plane may be disposed in the second plane. Also provided is an optical device for providing discrete undistorted images of the surface portion of the object when lying in the second plane and disposed in spaced registration with an object on the conveyor, a wide angle lens spaced from the surface portion of the object and having an axis generally normal to the second plane and a plurality of collecting mirrors spaced from and disposed at predetermined angular locations about the axis, the mirrors lying in planes generally parallel to the axis and for disposition between the wide angle lens and the object for reflecting light rays from the object through the wide angle lens and means for receiving the light rays passing through the wide angle lens from the object and the collecting mirrors representing an undistorted image of the object as viewed from each angular location with uniform focus of the three dimensional surfaces of the object.
In a further preferred embodiment according to the present invention, there is provided a method of optically inspecting an object, comprising the steps of conveying an object in a first predetermined direction with a surface portion of the object in a first plane, displacing a margin of the object in a direction generally normal to the direction of conveyance and independently relative to an opposite margin of the object to reorient the surface portion from the first plane into a second plane different from the first plane, providing a wide angle lens having an axis generally normal to the surface portion in the second plane and a plurality of collecting mirrors about the axis and between the lens and the surface portion and optically forming images of the surface portion of the object in the second plane from different angular locations about the axis with each image representing an undistorted image of the surface portion as viewed from each angular location with uniform focus of the three-dimensional surfaces of the object.
Accordingly, it is a primary object of the present invention to provide a novel and improved optical instrument, e.g., a vision head for visual inspection of predetermined areas of an object whereby undistorted perspective images of the object at different angular locations about the object are afforded.
It is another object of the present invention to provide a novel and improved conveyor for conveying an object for visual inspection wherein the predetermined areas of observation of the object may be tilted and levelled as needed for accurate observation by displacement of the side sections of the conveyor carrying the margins of the object relative to and independently of one another.
It is a still further object of the present invention to provide a novel and improved visual inspection machine affording undistorted perspective images of objects under observation at different angles about the objects for comparison with predetermined information whereby irregularities in the observed surface can be ascertained.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.